1. Field of the Invention
The present invention relates to a method of and an apparatus for producing an overlapping image area so as to eliminate a dropout due to registering mismatch in printing.
2. Description of Related Art
Four printing plates for yellow (Y), magenta (M), cyan (C), and black (K) inks are generally used for color printing. Although color printing machines are highly accurate and precise, high-speed rotary press may cause registering mismatch of approximately +0.05mm between the four printing plates. Various methods have been proposed in preparation of printing plates to eliminate such registering mismatch and maintain the quality of printed matter.
FIG. 1A is a conceptive view illustrating an image including two color areas CA1 and CA2 adjacent to each other.
Now the color of each area, shown by C[color area ID], is defined by dot% Hy, Hm, Hc, and Hk of yellow (Y), magenta (M), cyan (C), and black (K) as follows: EQU C[color area ID]=(Hy, Hm, Hc, Hk).
The colors of the color areas CA1 and CA2, shown in FIG. 1A, are expressed by: EQU C[CA1]=( 0, 80, 0, 0); and EQU C[CA2]=( 0, 0, 80, 0).
FIG. 1B shows an example of a defective image due to registering mismatch. In the image of FIG. 1B, the M plate is shifted in X direction by -0.05mm and the C plate by +0.05mm. The image includes two shift areas SA1 and SA2 in addition to the two color areas CA1 and CA2. The colors of the shift areas SA1 and SA2 are expressed by: EQU C[SA1]=( 0, 0, 0); and EQU C[SA2]=( 0, 80, 80, 0).
The shift area SA1 is a so-called dropout where no inks are applied, and which is conspicuous and deteriorates printed matter.
overlapping process is generally performed in prepress to prevent such deterioration. The overlapping process is to modify the shapes of two adjacent areas at their boundary to thereby make part of these areas overlap each other.
FIGS. 2A through 2C are explanatory views illustrating the overlapping process for the image of FIG. 1A.
FIG. 2A shows a first color area CA1a after the shape modification. The modified color area CA1a surrounds a white area which is smaller than that of the original color area CA1 shown by a broken line.
FIG. 2B shows a second color area CA2a after the shape modification. The modified color area CA2a is larger than the original color area CA2 shown by a broken line.
FIG. 2C shows a resultant image printed by overlaying the modified color areas Ca1a and CA2a one upon the other; thus an overlapping area LA is produced therein.
The width of the overlapping image area LA is determined such that a dropout can be prevented due to registering mismatch. For example, when the registering mismatch is in the range of -0.05 mm to +0.05 mm, the overlaid LA has width of approximately -0.1 mm. The overlaid area LA acts as a margin between the corrected color areas CA1a and CA2a and effectively prevents a dropout due to registering mismatch.
The overlapping image area LA is thin across and has a color similar to both of the adjacent color areas CA1a and CA2a, thus being unobtrusive and preventing deterioration of the resultant image as empirically proved.
There are two methods generally applied to the overlapping process.
The first is an optical overlapping method. An image area of a uniform color, called a tint, is prepared with a mask and a tint sheet, and an overlapping image area is created by adjusting the size of the masked area. This method, however, requires some materials like a mask and a tint sheet and is thus costly.
The second method, which was recently proposed, is processing of image data with an image processor such as a page make-up system.
In the conventional overlapping process with the image processor, however, an operator needs to specify the width and the color of an overlapping image area for each image element such figures and characters. When an image includes a large number of image areas, such specification of the width and the color is both labor and time consuming.